Torkstress hos urbana träd i Göteborg: Effekter av låg vattentillgång under torkan 2018

Sammanfattning: During late spring and most of the summer in the year of 2018 the weather in Sweden was more extreme than normal, with high temperatures and few rainfalls. This resulted in a drought, of which the effects were noticed all over the country. One of the noticeable effects was an earlier leaf abscission from trees. However, the effects of drought in an urban area are quite different compared to other environments. Something that is not very well studied is the effects of drought on vegetation growing in cities. The city environment is different compared to the tree’s natural environment, with its hard surfaces and buildings covering the sunlight and trapping extra heat due to various physical phenomena. This type of environment has various impacts on the trees, for example low infiltration of water into the soil surrounding the roots, which often result in water stress. Trees in turn affects the local climate, and ultimately the people living the city by the ecosystem services they provide. The impact on humans is one reason among many to take interest in the studies of the effects of drought on urban trees. Different species of trees have different characteristics, among them varying water use strategies and photosynthetic activity. Therefore, it is important to choose the species and the planting method carefully when planting new trees in the rough environment of a city. With these challenges in mind, this study was designed to study the effects of drought on urban vegetation. The main question was which trees experienced a negative effect of drought, and more specifically if the species and permeability of the surface under the tree crown influenced the trees’ ability to mitigate the drought in 2018. The method consisted of a combination of field-based measurements and remote sensing data analyses on a sample of urban trees in the Swedish city of Gothenburg, chosen with random sampling from a public database, based on certain criteria such as species, age and permeability of the surface covered by the tree crown. This resulted in a sample with a size of 44 individual trees, 23 of the species Tilia europaea and 21 of the species Quercus robur which were growing with different fractions of permeable surface below the tree crown (high ≥50% and low <50% respectively). The remote sensing consisted of a comparison of the data from the field studies with data on the change in the Normalized Difference Vegetation Index, NDVI (NDVI). NDVI displays the greenness or relative biomass observable from above. The comparison was done to estimate effects linked to the drought between late July 2017 and 2018. The results from the study showed a significant negative change in NDVI for all studied trees (mean = -0,017) (p = <0,1). Also, a significant interaction between species and surface permeability was found, with the largest NDVI decline occurring in Quercus robur with a high fraction of permeable soil under the tree crown (p = 0,029). No general difference in effect on NDVI could be proven between the species Tilia europaea and Quercus robur. Neither a general significant difference between NDVI for high and low fraction of permeability of the surface below the crown was shown. Both of these results differ from previous studies. The result from this study is a good example for showing the complexity that comes with studying urban trees, and the need for the development of better methods for assessing this issue.